A sublimator is a primary cooling device for the Extravehicular Mobility Unit (EMU) spacesuit. The sublimator includes a metallic porous plate that is exposed to space vacuum on one side. It is supplied with expendable feed water on the other side. The feed water freezes on the porous plate surface. The vacuum side progressively sublimes the ice to the vacuum of space to remove waste heat from the astronaut that is introduced into the plate.
The feed water may contain trace organic compound contaminants, typically from wetted non-metallic materials in the feed water loop (primarily amphipathic long-chain organic acids, fatty acids and surfactants with carboxylate functional groups) that can deposit on the metallic plate. Such a deposit may be in the form of an adherent molecular monolayer. This monolayer can impede sublimation and, therefore, debit sublimator performance.
The organic compounds may include processing aids, as exemplified by abietic acid, from EMU Neoprene Latex feed-water bladders, sodium dodecyl benzene sulfonate (a candidate soap for International Space Station-ISS processed water), acrylic acid oligomers from the ISS Water Processor multi-filtration bed sorbents, or other amphipathic compounds.
It is believed that the hydrophilic “heads” of the amphipathic compounds anchor or adhere to available cationic charge on the metallic surface of the porous plate, and that the hydrophobic “tails” of the amphipathic molecules form an intertwined monolayer. This phenomenon does not occur with non-volatile contaminants, such as corrosion products, iodine and biofilm, which do not have the structural features of an amphipathic compound. These non-amphipathic contaminants readily dislodge from the effluent side of the porous plate during the sublimation process and have minimal effect on performance.